Dynamic gating of polymers for isotropic properties

ABSTRACT

A polymeric article having isotropic properties is fabricated from anisotropic materials by orienting the polymer, as well as any fillers, in different directions at different planes through the polymeric article. In a first embodiment, molten polymer is passed between movable gate members (32 and 34) into cavity (30) of mold (28). The movable gate members (32 and 34) impart a strain on the molten polymer by moving in opposite directions (33 and 35) while the molten polymer is dispensed into the cavity (30). Reciprocating the movable gate members (32 and 34) yields a herringbone pattern. Having at least one movable gate member (38) have comb-like projections (40) can assure the strain is imparted deeper into the thickness of the polymeric article. In a second embodiment, molten polymer is passed through movable gates (96 and 100) on perpendicular sides of a mold cavity (94 and 98) where one movable gate (96) is located towards the top of the mold (84) and the other movable gate (100) is located towards the bottom of the mold (84). The gates (94 and 98) are simultaneously driven in directions (68 and 70) along their respective cavity sides while injecting polymer into the cavity. The finished article has a top layer (56) has polymer oriented (58) in a first direction and a bottom layer (52) with polymer oriented (54) in a perpendicular direction. Polymer is randomly oriented (62) between the top and bottom layer (56 and 52).

This application is a division of application Ser. No. 07/806,499, filedon Dec. 13, 1991, now U.S. Pat. No. 5,244,378.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to methods and apparatus forproducing molded parts having isotropic or nearly isotropic propertieswhen the starting material has anisotropic properties and, moreparticularly, to injection molding processes and apparatus for formingpolymeric items and/or products having superior mechanical, electrical,thermal, and chemical properties. Such items include but are not limitedto substrates, printed circuit boards, printed wiring boards,connectors, interposers, and the like.

2. Description of the Prior Art

The problems associated with anisotropic materials are well known bymanufacturers and users of polymeric materials. Attempts to overcomeanisotrophy include filling and/or loading the materials with variousfillers thereby restricting shrinkage or directional flow of the polymeras it is filling the mold cavity or being extruded. Successful use ofthese techniques has been limited and, in some cases, the techniqueshave actually aggravated the problems associated with anisotrophy.

Anisotrophy can and does affect numerous properties of a materialincluding mechanical, thermal, electrical, and chemical as well asmanufacturing or fabricating the material into a useful product. In themanufacture of an item such as a substrate used to attachsemiconductors, circuits, via-connectors, and the like, it is desirableto have properties the same in all directions if possible; that is, thesubstrate should be isotropic in nature. Many isotropic materials,however, possess poor physical properties that do not meet electricalsubstrate/board requirements.

Important properties to insure success of a molded polymeric item suchas a substrate for electrical components include (1) matched (withinallowable tolerances) coefficient of linear expansion to the materialsand components being used, (2) high thermal conductivities, (3) highcontinuous use temperature, (4) non-flammable, (5) high chemicalresistance, (6) low water absorption, (7) non-corrosive, and (8)processability. Additionally, it is preferable that the materials be lowcost and easy to manufacture into required geometries, be capable ofsurface mounting and vapor phase soldering technologies, be circuitizedwith various metals and by various techniques, and have long termmechanical stability.

A typical material having desirable properties for electrical substratesis a group of materials called liquid crystal polymers. However, incurrent injection molding processes, parts produced using thesematerials exhibit a uniaxial, anisotrophic orientation. Such partsexhibit properties which are highly dependent on orientation.

Recently, molding techniques have been developed which allow theproduction of parts having defined layers of directionally orientedmaterials. In particular, U.S. Pat. No. 4,994,220 to Gutjahr et al.discloses a process for injection molding parts from plasticized liquidcrystal polymer materials wherein a molten flow of the materials areinjected into a mold cavity through at least two different gatespositioned at different locations around the cavity at differentheights. Gutjahr et al. specifically points out that there areadvantages in having fibers in adjacent layers oriented 90° apart. Thatis, when layers of liquid crystal polymers are staggered at 90°, the endproduct is more isotropic since there will be criss-crossing lines ofanisotrophy. The method of Gutjahr et al. does not employ movable gatesto produce the oppositely oriented layers in the liquid crystal polymerparts and does not consider the effects of filling pressures and dropson the shinkage in multiple dierections. Other examples of newer moldingtechniques are found in U.S. Pat. No. 4,925,161 to Allan et al. whichshows a process for molding directionally-orientable materials usingshear force and in Kirkland et al., "New molding methods increase designfreedom", Plastics World, pp. 37-42 (February, 1991) which discusses the"live-feed injection molding" described in U.S. Pat. No. 4,925,161 toAllan et al. as well as a push-pull injection molding technique.

SUMMARY OF THE INVENTION

It is an object of this invention to provide methods and apparatus formolding parts with nearly isotropic properties from materials that arenormally anisotropic which employ at least one moveable gate associatedwith the mold.

It is another object of this invention to provide a multi-axiallyoriented thermotropic, liquid crystalline polymeric product exhibitingisotropic properies.

According to the invention, a thermally processable polymer material,such as a thermotropic liquid crystalline polymer, is molded in a mannerwhich orients the polymer material (e.g., polymer fibrils or the like)or fibrous or filler materials blended with the polymer material alongmultiple axes. In a first embodiment, the polymer material is flowedthrough a dynamic gate having spaced apart, opposed surfaces whichtranslate or move in different directions. As the melted polymermaterial traverses the moving, spaced apart, opposed surfaces, anangular shear is imparted to the polymeric material. Velocity ofmaterial flow, speed of gate surface movement in opposite directions,and rate of part solidification imparts an angular orientation to theopposite surfaces of the part being molded. Optimizing this combinationof material flow, gate movement, and part solidification rate providescross ply or biaxial properties to opposite planes of the part with atransition between surfaces. Typically, a 45° cross orientation, top andbottom, to the flow of material provides optimum properties; however,any tailoring desired can be made by varying the dynamics of theprocess. Enhancements of this first embodiment include driving the gatesurfaces in a reciprocating manner (not necessarily cyclic), whereby thepolymer orientation imparted by the moving gate surfaces resembles aherringbone pattern, and making at least one of the gate surfaces have acomb-like structure, whereby the shearing action caused by theoppositely moving gate surfaces can be extended further into thethickness of the part. In a second embodiment, the polymer material isflowed through at least two dynamic gates which are oriented 90° apartaround the perimeter of the mold cavity and at different heights. Themelted polymer material is simultaneously deposited through both gatesas both gates are moved across their respective sides of the moldcavity. The two dynamic gates may either be driven across theirrespective cavity sides by an external drive or by the melt flow itself.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a block diagram, partially in cross-section, showing a typicalinjection molding apparatus;

FIG. 2 is an isometric exploded view of sliced planes of a normalmolding with orientation in the flow direction;

FIG. 3 is an isometric view, partly in cross-section, of a substratecavity in a mold with the top removed showing the top and bottom surfacemembers of a dynamic gate according to a first embodiment of the presentinvention;

FIG. 4 is a cross-sectional side view of the substrate cavity as well asthe top and bottom surface members of the dynamic gate of FIG. 3;

FIG. 5 is an isometric exploded view of sliced planes of a molding withmulti-axis orientation produced according to one aspect of theinvention;

FIG. 6 is an isometric exploded view of sliced planes of a molding withmulti-axis orientation, and herringbone configurations, according toanother aspect of the invention;

FIG. 7 is an isometric view, partly in cross-section, of a substratecavity in a mold with the top removed showing the top and bottom surfacemembers of a dynamic gate according to an enhancement of the inventionshown in FIG. 3;

FIG. 8 is an isometric view of a part produced according to a secondembodiment of the invention illustrating the directions of polymerorientation at different levels of the part;

FIG. 9 is an isometric exploded view of sliced planes of a moldingproduced in a manner similar to the second embodiment of FIG. 8;

FIG. 10 is an isometric view, partly in cross-section, of a substratecavity in a mold with the top removed showing two dynamic gates locatedon different sides of the mold cavity and at different heights relativeto the mold cavity; and

FIG. 11 is an isometric view of a mold according to the secondembodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In isotropic materials, properties at any given point are the sameindependent of direction of measurement; whereas, anisotropic materialsdepend on the direction in which they are measured and on the symmetryexisting in the material. Anisotrophy may be considered analogous to theproperties associated with and across the grain of a piece of wood. Itis well known that by taking laminar layers of wood, which is highlyanisotropic, and laminating at cross or other directions, a product suchas plywood with resulting multi-directional properties can be obtained.Likewise, by taking an anisotropic polymer and molding it with laminarmulti-directional and/or merged layers, an isotropic material can beapproximated. This invention is directed to producing structuralmulti-axial orientation in a molded part by either introducing a dynamiccounter surface gate shearing of an injected polymer or composite as itflows through oppositely moving gate surfaces or by creating alternatinglayers of oriented polymer using two or more dynamic gates positioned atdifferent locations around the perimeter of a mold cavity. The inventionmay be used to mold polymers which are inherently anisotropic, such asliquid crystal polymers, or composite materials having fibers orfillers.

Referring now to the drawings, and more particularly to FIG. 1, there isshown a typical injection molding apparatus 10 wherein polymericmaterial, such as liquid crystal polymers, in granular form is fed intoa hopper 12. The polymeric material is fed by a feed screw 14 through aheating cylinder 16 to melt the polymer. The feed screw 14 is rotated bya screw motor 18 via a coupling 20. The feed screw 14 also functions asa hydraulic ram which is reciprocally moved back and forth by hydrauliccylinder 22 when a predetermined amount of material, as detected by thepressure within the cylinder 16, accumulates in front of the screw 14.The melted polymer is then forced through the nozzle 24 and into themold cavity 26 where it is held, under pressure, until it solidifies.The mold is then opened, the part removed and the process repeated. Themold cavity 26 could be duplicated at several locations in the mold 28such that multiple parts could be produced simultaneously.

The molding system of FIG. 1, or many other molding systems, could beused within the practice of the present invention which is primarilyconcerned with the point of polymer flow entry into the mold cavity 26.

FIG. 2 shows a series of planes through a molded part produced by aconventional static gate injection molding apparatus. The orientation ineach is in the direction of flow through the static gate, therebyresulting in a highly anisotropic structure.

FIGS. 3 and 4 show a mold with a dynamic gate according to oneembodiment of the present invention. The mold 28 has a cavity 30therein. The cavity 30 is simply a defined volume, much like cavity 26in FIG. 1, which is designed to hold melted polymer until it solidifies,afterwhich the molded part is removed from the cavity 30 by separatingthe mold 28. The dynamic gate is comprised of two spaced apart, movablegate members 32 and 34 which are positioned to one side of the cavity30. Melted polymer is flowed through feed line 36 past the movable gatemembers 32 and 34 and into the cavity 30. The movable gate members 32and 34 allow for creating nearly isotropic parts from anisotropicmaterials such as liquid crystal polymers. The movable gate members aredriven externally by a rack, solenoid, piston or the like, in thedirections indicated by double headed arrows 33 and 35, and are timedwith the flow of the polymer or composite. By driving the gate members32 and 34 in opposite directions, a shear force is introduced to the topand bottom surfaces of the polymer, causing orientation to the melt flowimmediately prior to its introduction into the cavity 30.

With reference to FIGS. 5 and 6, depending on the manner by which thegate members 32 and 34 are driven, the resulting parts can have somewhatdifferent configurations; however, the overall concept of producing amulti-axial part is the same. In FIG. 5, the resulting product isproduced by driving gate members 32 and 34, each only one way in anopposite direction 33 and 35 (i.e., drive gate member 32 one way anddrive gate member 34 the other way, and do not switch directions of thegate members 32 and 34 while forming the part), so that it has aplurality of planes therethrough which preferably are oriented from +45°in the top plane to -45° in the bottom plane. Hence, the orientations inthe top and bottom planes are oriented perpendicular to one anotherwhich will guarantee the molded product of having isotropic-likeproperties. Some intermediate planes through the product will have anorientation above or below 0° which depends on the depth to which theshearing influence of gate member 32 or 34 is exerted. Intermediateplanes through the product in the central region where no shearinginfluence is exerted or where the shearing influence of gate members 32and 34 is in balance will have an orientation at 0° like that of theplanes shown in FIG. 2. Hence, the planes through the product will havemultiple axes of orientation throughout the product from top to bottom.In FIG. 6, the resulting product is produced by driving gate members 32and 34 in opposite directions 33 and 35 in a reciprocating fashion(i.e., drive gate member 32 in one direction along 33 for a short timethen in the opposite direction along 33 for a short time, likewise forgate member 34) so that a wave pattern configuration, such as aherringbone pattern, of the polymer, fibrils or matrix materials,results in the planes throughout the product from top to bottom. Asdescribed in conjunction with FIG. 5, the degree of shearing influenceof gate members 32 and 34 on the polymeric material will vary dependingon the proximity to either surface; therefore, the product will havemultiple axes of orientation. The herringbone pattern configuration ofFIG. 6 provides additional isotropic-like properties since the planesthrough the product will themselves have opposite lines of anisotrophy.

The end product shown in FIGS. 5 and 6 is a function of the velocity ofpolymer flow through feed line 36, the speed at which the gate members32 and 34 translate (move or reciprocate), and the rate at which theproduct solidifies in the mold. All of these parameters are adjustableby manufacturer for purposes of optimizing the parts produced. If theflow across the cavity 30 is unduly influenced by the opposinghorizontal surfaces of the interior of the cavity, the relationshipsbetween flow rate, gate movement and solidification rate can be adjustedto compensate. Alternatively, a volume controlled short shot can beinjected followed by a compression of the cavity to the final dimensionsof the part.

FIG. 7 shows an alternative mold configuration to that shown in FIG. 3wherein lower gate member 34 has been replaced with a gate member 38having a comb-like configuration. It is also possible to replace gatemember 32 with a comb-like structure. The operation of gate members 32and 38 is similar to that described above in conjunction with FIGS. 3-6.The principal advantage of using a gate member 38 with comb projections40 is that the shearing forces can be exerted deeper into the part bythe projections 40. Therefore, the end product will have greatermulti-axial orientations in its mid-region and less transitional regionsthan could be achieved with the smooth gate members 32 and 34.

The sequence of events in the molding process are to close the mold 28,flow the polymer or composite material to and past the oppositely movinggate members 32, 34 and/or 38, solidify the molded part, eject the part,and repeat the cycle for the next part. Although only one cavity 30 isshown in the illustrative preferred embodiment, it will be understood bythose skilled in the art that multiple cavities may be implemented inthe mold 28, thereby increasing production of parts. While liquidcrystal polymers are preferred for a specific application of theinvention, it will be apparent that other materials and composites maybe used as may be appropriate to the application.

FIG. 8 illustrates a molded part 50 produced according to a secondembodiment of the invention which can have a lower layer 52 havingpolymer material oriented according to arrows 54, an upper layer 56having polymer material oriented according to arrows 58, and a middlelayer 60 having polymeric material randomnly ordered as indicated byarrows 62. The molded part 50 is made by simultaneously injecting meltedpolymer through gate areas which are represented by blocks 64 and 66.The gate areas 64 and 66 are obviously not associated with molded part50; however, they are indicated in FIG. 8 to illustrate how the moldedpart 50 filling is initiated. The mold itself is discussed below inconjunction with FIGS. 10 and 11. Gate area 64 is located in the moldcavity at a point positioned to form the lower layer 52 of the moldedpart 50 and traverses a path 68 along one side wall of the mold cavity.Gate area 66 is located in the mold cavity at a point positioned to formthe upper layer 56 of the molded part 50 and traverses a path 70 alonganother side wall of the mold cavity. Melted polymer is injected throughgate area 64 in direction 72 and gate area 66 in direction 74 as thegate areas move along paths 68 and 70, respectively. Injecting themelted polymer through narrow gate areas 64 and 66 creates uniaxialorientation of the polymer delivered and that uniaxial orientation issolidified into the molded part 50 such that it has lower and upperlayers 52 and 56, respectively, that have opposite axes of orientation(e.g., 54 and 58, respectively). Hence, the molded part hasisotropic-like properties even though it may be molded from anisotropicmaterials. In the intermediate region between the gate areas 64 and 66,the direction of polymer flow 72 and 74 does not contribute greatly tothe ultimate orientation of the polymer fibrils, filler or the like;therefore, the middle layer 60 has portions which have randomorientation 62. The thickness of the middle layer 60 can vary with thepositioning of gate areas 64 and 66.

As described above in conjunction with FIGS. 3-7, the molded part 50 canbe made using anisotropic materials, such as liquid crystal polymers,polymers with associated filler or fiber materials, or the like. Thesequence of events is to close the mold, fill the mold with meltedpolymer using dynamically moving gate areas 64 and 66, solidify thepart, eject the part, and repeat.

FIG. 9 illustrates that a molded part which embodies the principlesdescribed in conjunction with FIG. 8 can be produced using multiple gateareas. In FIG. 9, layers 76, 78, 80, and 82 of the molded part have astaggered orientation with respect to one another. This is achieved byproviding the mold with two gate areas similar to 64 and two gate areassimilar to 66, both of which are discussed in conjunction with FIG. 8,on different sides of a mold cavity. The optimum number of repetitionsof the staggered pattern is determined by the manufacturer. A particularapplication of a multilayered polymer part would be for multilayeredcircuits.

FIGS. 10 and 11 show a mold 84 which may be used to make the molded part50 of FIG. 8. The mold 84 has top and bottom halves 86 and 88,respectively, which are aligned together by pins 90 and bushings 92. Thetop half 86 has an upper cavity 94 and dynamic gate 96. The bottom half88 has a lower cavity 98 and dynamic gate 100. Dynamic gates 96 and 100each have blade runner areas 102 and 104, respectively. Each of whichhave tunnels 106 and 108 therethrough. Melted polymer 110 fills theupper and lower cavities 94 and 98 by directing polymer through thetunnels 106 and 108 as the dynamic gates 96 and 100 move in directions112 and/or 114. With reference back to FIG. 8, tunnels 106 and 108correspond to gate areas 66 and 64, respectively. FIG. 10 illustratesthat the dynamic gates 96 and 100 can be passively driven by the meltedpolymer 110 impinging on drive surfaces 116 and 118, respectively, whilethe polymer 110 is extruded through tunnels 106 and 108. Alternatively,FIG. 11 illustrates that the dynamic gates 96 and 100 can be driven byexternal sources 120 and 122, respectively, such as by a rack, piston,or some other mechanism.

A particular advantage of the mold illustrated in FIGS. 10 and 11 andits operation described in conjunction with FIG. 8 over the molddescribed in U.S. Pat. No. 4,994,220 to Gutjahr et al. is that by usingdynamic gates 96 and 100, the same kinds of filling pressures and dropswill be encountered as melted polymer 110 fills the cavities 94 and 98.Therefore, the molded part 50 will have equal shrinkage factors indifferent directions 54 and 58. Gutjahr et al. uses repeating cyclicfeeding of the polymer, while the present embodiment of the inventionuses ongoing simultaneous feeding to feed the cavity.

While the invention has been described in terms of its preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with considerable modification within the spirit andscope of the appended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A process for forming apolymeric article comprising the steps of:supplying a melted polymermaterial to a mold having a restricted gate area comprised of at leasttwo spaced apart opposing gate members which provides access to a cavityin said mold; and subjecting said melted polymer material to a shearforce by moving each of said two opposing gate members in a directionperpendicular to a flow path of said melted polymer material from asupply into said cavity in said mold.
 2. A process as recited in claim 1wherein both of said two opposing gate members move in oppositedirections during said subjecting step.
 3. A process as recited in claim2 further comprising the step of reciprocating a direction of movementfor both of said two opposing gate members during said subjecting step.4. A process for forming a polymeric article; comprising the stepsof:supplying a melted polymer material to a mold having at least twomovable gates positioned on different sides of a cavity in said mold andat different height regions with respect to a height dimension acrosssaid cavity in said mold; passing said melted polymer material throughapertures in each of said two movable gates into said cavity of saidmold; and moving each of said two movable gates along its respectiveside of said cavity during said passing step.
 5. A process as recited inclaim 4 further comprising the step of positioning said two movable gatemembers to move on paths perpendicular to each other.
 6. A process asrecited in claim 4 wherein said step of moving is performed passivelyunder an influence of a flow of said melted polymer material,
 7. Anprocess as recited in claim 4 wherein said step of moving is performedactively by a means for simultaneously moving each of said two movablegates.